1. Field
Systems and methods for providing antenna calibration can be used in a variety of applications. A method of calibrating an antenna array for use in a traffic advisory system, sometimes referred to as a traffic awareness system, or traffic alert and collision avoidance system provide a mechanism that renders complex combining circuitry, beamforming circuitry, and internal calibration circuitry unnecessary in the array.
2. Description of the Related Art
There are various techniques for calibration of antennas using internal sources. Calibration is generally accomplished using an internal circuit which is capable of generating a test signal of a known equivalent bearing that is then received by the elements. The phase relationship between the signals received by the elements is a function of bearing. Consequently, variable and unknown phase constants (antenna cables, receiver front end differences, and the like) can be compensated out of the equation.
U.S. Pat. No. 5,469,172 discusses an internal calibration apparatus used to inject a “known” phase signal into the elements. This known phase signal may correlate to a particular bearing angle. Subsequent bearing measurements may be made relative to this reference to get absolute bearing.
U.S. Pat. No. 4,855,748 discusses an additional method of injecting a self test calibration signal to aid in calibrating out phase errors in the receive system.
U.S. Pat. No. 5,191,349 discusses reviewing the means by which the TCAS directional antenna accomplishes bearing detection.
U.S. Patent Application Publication No. 2006/0009909 discusses means for using positional data of both an intruder and a host aircraft to provide a correction factor to the measured bearing data so as to more nearly perfect the data. However this may still rely on an internal beam forming network to determine the initial bearing estimate.
Another approach for antenna calibration may include locating the antenna very close to the receiver and connecting the two with a very short, precisely manufactured set of matched cables. Thus, the phase lengths between the antenna elements and the receivers may be ideally equal. However, this approach does not compensate for aging and temperature variations that do not affect each channel identically.